1. Field of the Invention
This invention relates to a furnace and method for heating glass sheets that each have a coated surface and an oppositely facing uncoated surface.
2. Background Art
Glass sheets are heated for processing such as forming, quenching for heat strengthening or tempering, or forming and quenching, etc. Such heating is conventionally performed by either electric resistance-type furnaces or by forced convection furnaces utilizing gas burners. Prior electric resistance-type furnaces are disclosed by U.S. Pat. No. 3,934,970 McMaster et al., U.S. Pat. No. 3,947,242 McMaster et al., and U.S. Pat. No. 3,994,711 McMaster, all of which have lower and upper housing portions that support electric resistance elements that provide radiant heating of glass sheets from below and above at their lower and upper surfaces during conveyance within a heating chamber of the furnace housing. Prior gas burner type forced convection furnaces are disclosed by U.S. Pat. No. 4,738,705 McMaster and by the U.S. patents of Kenneth R. Kormanyos: U.S. Pat. Nos. 5,669,954; 5,672,191; 5,735,924; 5,762,677; and 5,792,232, all of which have lower and upper gas burner type forced convection heaters that provide forced convection heating of conveyed glass sheets from below and above at lower and upper surfaces of the glass sheets. The U.S. Pat. No. 6,131,411 Schnabel, Jr. and U.S. Pat. No. 6,279,349 Schnabel, Jr. disclose a glass sheet heating furnace having a lower housing portion including electric resistant heating and an upper housing portion including gas burner type forced convection heating.
Air jets provided by compressed air have also previously been used to entrain heated air within a furnace to provide glass sheet heating. Such air jets are disclosed in U.S. Pat. No. 4,505,671 at the exit end of an electric resistant type furnace to provide planarity of the heated glass sheets. Also, U.S. Pat. No. 4,529,380 McMaster discloses such air jets, which are heated by an external heater to provide the primary source for heating the furnace.
Electric resistance furnaces for heating glass sheets are less expensive to manufacture than gas burner type forced convection furnaces and also can be more easily controlled by less involved control apparatus. In addition, electric resistance furnaces for heating glass sheets also result in a greater percentage of the energy input being transferred into the glass for the heating. However, electrical energy is more expensive than natural gas heating and the radiant heating involved cannot efficiently heat low-emissivity coated glass sheets. In fact, some reflective coatings reflect over 90% of incident radiant heat which makes it virtually impossible to efficiently heat such glass sheets by radiant heat with electric resistance-type furnaces.
An object of the present invention is to provide an improved furnace for heating coated glass sheets.
In carrying out the above object, the furnace of the invention heats glass sheets that each have a coated surface and oppositely facing uncoated surface. A housing of the furnace defines a heating chamber an d h as entrance and exit ends, and a roll conveyor of the furnace conveys glass sheets to be heated within the heating chamber along a horizontal conveying plane between the entrance and exit ends of the furnace with the coated surface facing upwardly and with the uncoated surface facing downwardly and supported by the roll conveyor. Electric resistance elements of the furnace heat the housing within the heating chamber below and above the conveying plane. A hot air distributing system of the furnace is located within the heating chamber between the entrance and exit ends above the roll conveyor and supplies hot air jets downwardly toward the upwardly facing coated glass surface of each conveyed glass sheet. These hot air jets also entrain hot air within the heating chamber and the combined flow of hot air provides convective heating of the coated glass surface in addition to the radiant heating thereof by the electric resistance elements. A control of the furnace increases the forced convection heating of the glass sheet by the hot air distributing system during the glass sheet conveyance to balance the heating and the resultant temperature of the upwardly facing coated surface and the downwardly facing uncoated surface of each conveyed glass sheet being heated.
The hot air distributing system includes an array of hot air distributors positioned above the roll conveyor between the entrance and exit ends of the furnace. A source of pressurized air is located outside of the furnace and supplies pressurized air to the hot air distributors. The hot air distributors include heat exchangers for heating the pressurized air prior to delivery therefrom as the downwardly directed hot air jets.
The furnace also includes a controller for terminating the supply of pressurized air to the hot air distributors below which there is no glass sheet being conveyed so as to thereby provide efficiency in the heating.
Each hot air distributor includes an upper manifold and a vertical support tube having an upper end that is supported by the upper manifold extending downwardly therefrom without direct fluid communication with the upper manifold. The support tube has a lower end adjacent the roll conveyor, and each hot air distributor also includes a horizontal delivery tube that extends in opposite directions from the lower end of the supply tube in fluid communication therewith and has downwardly opening delivery orifices. The heat exchanger of each hot air distributor includes a heat exchanger tube having an inlet that is fed pressurized air and an outlet through which pressurized air heated within the heat exchanger tube is fed to the vertical support tube for flow to the horizontal delivery tube and delivery through the orifices thereof as the downwardly directed hot air jets that entrain hot air within the heating chamber and provide convective heating to the upwardly facing coated glass surface of each conveyed glass sheet. The horizontal delivery tube has a pair of opposite lateral ends, and the heat exchanger tube has inclined portions that extend with an inverted V shape between the upper supply tube and the pair of opposite lateral ends of the horizontal delivery tube. More specifically, the heat exchanger tube includes a pair of inclined portions that extend with an inverted V shape between the upper supply tube and the pair of opposite lateral ends of the horizontal delivery tube, and the upper supply tube has a vertical portion that depends downwardly from the furnace housing and a horizontal portion that extends horizontally from the vertical portion thereof with each supply tube supporting a plurality of the hot air distributors.
In one construction of the hot air distribution system, each hot air distributor includes a pair of inclined supports having upper ends connected to the upper manifold and having lower ends connected to the horizontal delivery tube in an inverted V shape that provides support to the delivery tube. This embodiment also includes support brackets that connect adjacent hot air distributors at the lower ends of their inclined supports. These brackets have upper connectors, and the furnace housing has downwardly extending roof supports that support the upper connectors of the brackets which thereby cooperate in supporting the delivery tubes of the hot air distributors.
In two different embodiments of the furnace disclosed, the roll conveyor includes a drive that provides rotary driving of the conveyor. In one embodiment, the rotary driving is provided in opposite directions to provide oscillation of each glass sheet being heated during conveyance thereof in opposite directions between the entrance and exit ends of the furnace. In another embodiment, the drive provides rotary driving the conveyor in one direction to provide conveyance of each glass sheet in one direction from the entrance end of the furnace to its exit end.
The furnace is constructed with its housing including a lower portion having a flat floor and vertical side walls having upper ends, and the housing also includes an upper portion of a downwardly opening semicircular shape having lower ends located above the upper ends of the vertical side walls of the lower housing portion such that the housing portions cooperate to define the heating chamber. The electric resistance elements are mounted within the heating chamber on the lower housing portion floor below the roll conveyor and on the semicircular upper housing portion above the conveyor. The upper ends of the vertical side walls of the lower housing portion and the lower ends of the semicircular upper housing portion cooperate to define side slots, and the roll conveyor includes rolls having ends that project outwardly through the side slots of the furnace housing. Heat seals of the furnace seal between the lower housing vertical wall upper ends, the semicircular upper housing lower ends and the rolls to reduce heat loss from the heating chamber. The previously mentioned drive rotatively drives the roll ends externally of the heating chamber.
Another object of the present invention is to provide an improved method for heating glass sheets that each have a coated surface and an oppositely facing uncoated surface.
In carrying out the above objects, the method for heating coated glass sheets in accordance with the invention is performed by conveying each glass sheet on a roll conveyor along a horizontal conveying plane within a heating chamber of a housing between entrance and exit ends thereof with the coated surface thereof facing upwardly and with the uncoated surface thereof facing downwardly. Resistance elements are electrically heated at locations below and above the conveying plane to provide radiant heat to both the downwardly facing uncoated surface and the upwardly facing coated surface of each conveyed glass sheet. Hot air jets are supplied downwardly toward the upwardly facing coated glass surface of each conveyed glass sheet. The hot air jets entrain hot air within the heating chamber and the combined flow of hot air provides forced convection heating of the coated glass sheet in addition to the radiant heating thereof by the electric resistance elements. The forced convection heating by the downwardly directed hot air jets is increased during the glass sheet conveyance to balance the heat supplied thereto and the resultant temperature of the upwardly facing coated surface and the downwardly facing uncoated surface of each conveyed glass sheet being heated.
The hot air jets are supplied downwardly through an array of hot air distributors between the entrance and exit ends of the furnace with the hot air jet supply being terminated at the hot air distributors when there is no conveyed glass sheet below those distributors on the roll conveyor in order to provide efficiency in the heating.
The roll conveyor is rotatively driven to provide the glass sheet conveyance. In one practice, the rotational driving of the roll conveyor is in opposite directions to convey the coated glass sheet in an oscillating manner between the entrance and exit ends of the furnace for the heating. In another practice, the roll conveyor is rotatively driven in one direction to convey each coated glass sheet from the entrance end of the housing to its exit ends in a manner that is utilized for higher production glass sheet processing.
The objects, features and advantages and advantages of the present invention are readily apparent from the following detailed description of the preferred embodiments of the invention when taken in connection with the accompanying drawings.